Noble gases in SNC meteorites: Shergotty, Nakhla, Chassigny

Abstract

I report the elemental and isotopic composition of the noble gases in various fragments of the Shergotty, Nakhla, and Chassigny meteorites. Conclusions presented earlier (OTT and Begemann, 1985), which were based on results for trapped Ar, Kr and Xe only, are reaffirmed by the complete set of data. These include: 1. 1) Shergotty contains trapped gases that can be described as a mixture of the gas type found in the glass of shergottite EETA 79001 (SPB-type gases; possibly introduced by shock from the Martian atmosphere) and a second component characterized by low 36Ar 132Xe , 84Kr 132Xe and 129Xe 132Xe . Trapped gases found in Chassigny ( 36Ar 132Xe ~ 10 ; 84Kr 132Xe ~ 1 ; 129Xe 132Xe = 1.03 ) could be a pure sample of this second component. 2. 2) Trapped gases in Nakhla cannot be described as such a simple mixture. 3. 3) Xe in Chassigny has a solar-like isotopic composition, with 129Xe 132Xe significantly lower than in SPB-Xe (~ 1.03 vs. ~ 2.4). If these values are characteristic for interior and atmosphere, respectively, of the SPB, the evolution of the SPB and Earth must have differed greatly. Arguments are presented that Chassigny-type gases are a better choice than (fractionated) terrestrial air for the second component, that, when mixed with SPB-gases produces the Shergotty composition. The ratios 40Ar 36Ar t and 40Ar 129Xe ∗ (where 129Xe ∗ is 129Xe in excess of a 129Xe 132Xe ratio of ~ 1) in Shergotty agree with the respective values for EETA 79001 glass. This is taken as evidence for a late introduction of the supposedly shock implanted atmospheric component into Shergotty; it is at odds with scenarios that introduce the atmospheric component ~180 m.y. ago. Based on the variable ratio 40Ar 129Xe ∗ and its unshocked nature, Nakhla acquired its excess 129Xe by a different process. 21Ne-based cosmic-ray exposure ages agree well with values in the literature, but do not readily agree with the younger ages based on the abundance of cosmogenic 126Xe. Neutron capture products are present in Nakhla Kr and there is evidence for a fission-like component in Xe of both Shergotty and Nakhla. GCR interactions with the SPB are unlikely to be the reason for the low 36Ar 38Ar ratio in SPB-Ar.